Method of making a foraminous abrasive article

ABSTRACT

A method for manufacturing a foraminous abrasive article whereby abrasive particles are concentrated in an external surface region of a foraminous substrate. The method includes the steps of: providing a foamable, hardenable, liquid resin composition and a foraminous substrate having an external surface; foaming the resin composition effective to disperse a plurality of gas bubbles throughout the resin composition; applying the foamed resin composition to the external surface of the substrate to form a foam coating layer having an upper surface; applying a plurality of abrasive particles to the upper surface of the foam coating layer; heating the foamed coating layer effective to eliminate the gas bubbles from the foamed coating layer; and hardening the resin composition to attach the abrasive particles to the substrate. The invention also concerns the foraminous abrasive article products of this method.

The invention is generally related to a method of making a foraminousabrasive article with use of a labile foamed binder as a transitoryabrasive particle barrier by which abrasive particles are preferentiallybonded to external surface regions of a foraminous substrate.

BACKGROUND OF THE INVENTION

Foraminous abrasive articles have been made, for example, as nonwovenabrasive articles constituted of a network of synthetic fibers orfilaments which provide surfaces upon which abrasive particles areadhesively attached.

Nonwoven abrasive articles, in particular, are useful in variousconverted forms, such as wheels, sheets, discs, flap brushes, and thelike. In these converted forms, the resulting nonwoven abrasive articlesare useful to clean, condition, and/or decorate the surfaces suchmaterials as metal, wood, plastics, glass, ceramics, composites, and thelike. A particularly important use for such nonwoven abrasive articlesis to scuff automotive body finishes prior to the application of furthercoatings.

Conventional nonwoven abrasive articles generally involve a mat offibers which have on at least a portion of their surface an abrasivecoating comprising abrasive particles and a binder. As known, the fiberscan be formed from various synthetic polymers, including polyamides,polyesters, polypropylene, polyethylene, and various copolymers thereof.Also, naturally occurring fibers such as cotton, wool, bast fibers, andvarious animal hairs may also be suitable. Suitable abrasive particlescan be formed of flint, garnet, aluminum oxide, diamond, siliconcarbide, and the like. Binders commonly comprise cured versions of hideglue or varnish, or one or more resins such as phenolic,urea-formaldehyde, melamine-formaldehyde, urethane, epoxy, and acrylicresins. Phenolic resins include those of the phenol-aldehyde type.

In one conventional general scheme, nonwoven abrasive articles have beenmanufactured by applying to a nonwoven web starting material a "prebond"coating of binder precursor solution, which includes one or more of theabove-named resins, in order to impart sufficient strength to thenonwoven web starting material so that it can better tolerate subsequentprocessing. A "make" coating optionally has been applied to theprebonded nonwoven web and left non-fully cured up until the time whenabrasive particles are later applied to the web to help attach theabrasive grains throughout the lofty fibrous mat. Finally, an abrasivecoating of resinous binder material and abrasive particles has beenapplied onto the nonwoven to increase the abrasive characteristics ofthe nonwoven. The resin binder used in the various coatings may be thesame or different.

In a conventional approach, the binder coating for the abrasiveparticles has been applied to the nonwoven web as a non-foameddispersion of a binder resin and abrasive particles in a liquid medium.The dispersion is then applied to the nonwoven substrate by means, suchas spraying, that either atomizes the mixture to atomized droplets, orotherwise causes a film of the dispersion to be formed on the web. Theatomized droplets or film are applied to the nonwoven web and cured.During curing, the droplets either flow together (coalesce) byheat-induced viscosity reduction, or cure as individual droplets wherethey stand. Films usually flow together to cover (wet) most of thefilaments in the nonwoven web, and they are cured in position.

As the binder resin, phenolic resins are used extensively to manufacturenonwoven abrasive articles because of their thermal properties,availability, low cost, and ease of handling. The monomers currentlyused in greatest volume to produce phenolic resins are phenol andformaldehyde. Other important phenolic starting materials are thealkyl-substituted phenols, including cresols, xylenols,p-tert-butylphenol, p-phenylphenol, and nonylphenol. Diphenols, e.g.,resorcinol (1,3-benzenediol) and bisphenol-A (bis-A or2,2-bis(4-hydroxyphenyl)propane), are employed in smaller quantities forapplications requiring special properties.

There are two basic types of phenolic resins: resole and novolakphenolic resins. Molecular weight advancement and curing of resolephenolic resins are catalyzed by alkaline catalysts. The molar ratio ofaldehyde to phenolic is greater than or equal to 1.0, typically between1.0 and 3.0. In the production of adhesive coatings for nonwovenabrasives, one standard starting phenolic resin composition is a 70%solids condensate of a 1.96:1.0 formaldehyde:phenol mixture with 2%potassium hydroxide catalyst added based on the weight of phenol. Thephenolic resin composition is typically 25-28% water and 3-5% propyleneglycol ether, which are required to reduce the viscosity of the resin.In conventional techniques for making nonwoven abrasives, the phenolicresin has not been applied to the nonwoven web substrate in foamedcondition.

It is possible for nonwoven abrasive articles resulting from theseconventional coating methods to have an abrasive particle distributionthrough the thickness of the web such that abrasive particles somewhatmore concentrated near the external surface regions of the mat (due tospray coating). However, substantial quantities of abrasive particlesnonetheless are usually present throughout the web including theinterior regions. The abrasive particles lodged in the central interiorregions of the web are not immediately available for useful abradingwork until the external surface(s) of the article is worn or otherwiseattrited.

For applications in automotive body finishing, the ability of nonwovenabrasive articles to quickly create a uniformly scratched surface is ofprimary consideration. For this purpose, it would be advantageous tohave the abrasive particles concentrated at the external surfaces ofsuch nonwoven articles thereby increasing the number of such particlesin immediate, simultaneous contact with the workpiece, instead of beingmore or less uniformly distributed throughout the thickness of the web.

The state of the art can be further understood by reference to thefollowing references in particular:

U.S. Pat. No. 2,958,593 (Hoover et al.) describes a low density, open,nonwoven fibrous abrasive article formed of fibers formed into anonwoven web, abrasive particles, and a curable binder. Organic fibersare adhesively bonded together at crossing and contacting points, andabrasive particles are also adhesively bonded to the web fibers. Theinterstices between the fibers are left open and unfilled by adhesive orabrasive particles so that the web is non-clogging and non-filling innature, and it consequently can be readily cleaned upon flushing. Theadhesive used to bond the contacting points of the fibers in the web canalso be used as the means of attaching the abrasive particles to fibersin the web. The abrasive mineral particles are sprayed onto the nonwovenweb as dispersed in the liquid binder solution. Alternatively, themineral binder can be roll coated, dip coated, or separately appliedrelative to the abrasive particles, upon the nonwoven web. For instance,the mineral binder can be first sprayed upon the web followed by siftingof the abrasive mineral particulate upon the resin wetted web.

U.S. Pat. No. 3,175,331 (Klein) discloses a cleaning and scouring padcomprising one or more fibrous batts, heat-sealed so as to be capable ofhaving enclosed therein a solid washing composition, and in which theouter surface of the pad has grit adhered thereto to provide acontinuous, uninterrupted scouring surface extending over the entireouter surface of the pad. A fusible adhesive in liquid form is appliedonto either or both surfaces of the fibrous batt sufficient to lightlyimpregnate the outer surface only of the batt to bond outer fiberstogether while not filling voids between fibers or penetrating to fiberson the opposite surface of the batt. Therefore, the amount of adhesiveis regulated in order to concentrate the adhesive in the area of thesurface of the batt instead of the interior of the batt. Where scouringaction is desired, abrasive grit is pre-mixed into the impregnatingadhesive and applied to the surface(s) of the fibrous batt, such as byspraying. Where the finished pad is to be used for washing instead ofscouring, the abrasive material can be omitted from the impregnatingresin used in Klein.

However, in order to limit the penetration depth of the liquid resininto the batt as in Klein, careful and time-consuming pre-considerationand monitoring during processing of many parameters, such as resincoating amount, resin flowability, resin viscosity, web thickness, webdensity, and so forth, would be required.

French Patent Application Publication No. 2,409,095 discloses the use ofcollapsible, colored foams to concentrate pigments near a surface of aporous support, such as a fibrous support. The pigments or ink basematerials are incorporated into and intimately admixed with a binder indispersion with a mechanical foam machine. A colored foam is formed inwhich the pigment and binder resins are temporarily suspended within afoam. The resulting colored foam is applied to a fabric surface andsubjected to heat to collapse the foam such that the pigment and binderresidues only penetrate the fabric surface to a limited degree. Frenchpatent 2,409,095 thus allows for surface printing or dyeing of a side orboth sides of a fabric without strike-through problems. French patent2,409,095 teaches pigments, usually relatively light and soft materials,and not dense, solid granulate, as suspended, along with the binder,within and throughout the bulk of the foam. Therefore, pigments locatedat the lower regions of the foam layer will contact the fabric surfaceimmediately after, or very shortly after, the foam is applied to thefabric surface.

U.S. Pat. No. 4,969,975 (Biggs et al.) describes a process by which ahomogeneous sheet comprising a uniform distribution of fibers and/orparticles, which otherwise might float and/or settle, by incorporatingthe fibers and/or particles into a froth or foam itself, and depositingand draining the foamed dispersion on a fibrous support. As the particleadditives are distributed within and throughout the foam in Biggs etal., at least a portion of such additives will immediately come intocontact with the substrate used as the coating support.

As can be appreciated from the above, there still remains a need for atechnique to concentrate abrasive particles in the external surfaceregions of a foraminous abrasive article, such as a nonwoven, by asimple manufacturing scheme that does not require extensive preparationand monitoring during processing.

SUMMARY OF THE INVENTION

The invention is generally related to a method of making a foraminousabrasive article in which abrasive particles are bonded predominantly inthe surface regions of a foraminous substrate by use of a labile foamedbinder as a transitory abrasive particle barrier.

For purposes of this application, "foraminous" means porous to air.Foraminous substrate materials within the scope of the invention includefibrous webs, such as nonwovens and woven materials, and non-fibrousmaterials, such as cured, open cell, synthetic foams and natural spongematerials.

A foraminous abrasive article formed by the inventive method is endowedwith a high concentration of abrasive grains available at the surface ofthe substrate to deliver immediate, simultaneous polishing and/orabrading action while retaining an open, lofty, and flexible interiorconducive to rinsing/flushing.

For purposes of the present invention, by "labile", it is meant that afoamed condition imparted to a liquid dispersion of binder material issusceptible to undergo physical or chemical change and that the foamedstate of the binder dispersion is therefore transitory as it can becontrollably eliminated. Specifically, in the context of the presentinvention, the labile foam binder is thermally unstable and willcollapse ("fall") when heated as air and/or water is substantiallyevacuated from the foam in response to the heat and any concomitantcuring of the resin associated with heating. By the term "foam", it ismeant a dispersion of gas bubbles throughout a liquid where each bubbleis enclosed within a thin film of the liquid. The gas bubbles may be ofany size, from colloidal to macroscopic, with the proviso that the foamstill retains sufficient cohesion to form an elastic coating film. Thelabile foams of the invention thus also encompass "froths", which areunstable types of foam consisting of relatively large bubbles of gas.

Briefly and in general terms, the invention uses binder dispersionsapplied to a foraminous substrate as a labile foam to provide atemporary support and a physical barrier between the upper surface ofthe substrate and abrasive particles that are subsequently deposited onthe upper, exposed surface of the labile foam. The foam is thencontrollably collapsed by manipulation of the thermal environment of thefoam-coated substrate. As heat is used to destabilize the labile foam,air and water are substantially eliminated from the foam to cause thefoam to incrementally break down and "fall" until the binder resinsolids and the abrasive particles gently come to rest in the surfaceregions of the substrate. The binder is then fully solidified orhardened, such as by heat curing a thermosetting binder resin, to attachabrasive particles to surfaces of the substrate material. The substratematerial, e.g., a fibrous material, a cured foam material, a spongematerial, and so forth, constitutes the support matrix or skeleton ofthe foraminous substrate. The present invention enables the attachmentof the abrasive particles to be concentrated in the surface regions ofthe substrate.

Importantly, before the foam is completely collapsed, the dry abrasiveparticles as a matter of course are effectively pre-coated with sometacky binder resin by contact with the foam and afforded a soft landingon the surface of the substrate, such as a nonwoven web, when the foamultimately collapses upon the actual surface of the foraminoussubstrate. As a consequence, the need to use large amounts of liquidresin at the substrate surface to ensure retained contact and capture offalling abrasive particles before they can deeply penetrate into theporous substrate is effectively avoided by the present invention.

In one embodiment, the method of the present invention provides atechnique for manufacturing a foraminous abrasive article where abrasiveparticles are concentrated in an external surface region of a foraminoussubstrate by a scheme of steps, including:

(a) providing a foamable, hardenable, liquid resin composition and aforaminous substrate having an external surface;

(b) foaming the resin composition effective to disperse a plurality ofgas bubbles throughout the resin composition;

(c) applying the foamed resin composition to the external surface of thesubstrate to form a foam coating layer having an upper surface;

(d) applying a plurality of abrasive particles to the upper surface ofthe foam coating layer;

(e) heating the foamed coating layer effective to eliminate the gasbubbles from the foamed coating layer; and

(f) hardening the resin composition to attach the abrasive particles tothe substrate to form a foraminous abrasive article.

In one further embodiment of the invention, the foraminous substrate isa fibrous substrate. Upon completion of this embodiment of the inventioninvolving a fibrous substrate, preferably at least about 80%, by weight,of the abrasive particles deposited on the labile foam coating layerbecome attached (bonded to fibers) on or in the fibrous substrate atlocations within a vertical distance measured from the coated externalsurface that is no greater than about 25%, more preferably no greaterthan about 15%, of the overall average thickness of the fibroussubstrate. The vertical distance and thickness of the fibrous substrateare each measured in a direction normal to a horizontal plane defined bythe external surface of the fibrous substrate. Therefore, for a nonwovenweb having an overall thickness of 10 mm, at least about 80% by weightof the abrasive particles applied to the upper surface of the foamedcoating layer ultimately become bonded to the web fibers located withina vertical distance of 2.5 mm from the external surface of the web thatwas coated with the foamed coating layer. The fibrous substratepreferably is a nonwoven web; although other fibrous substrates withporosity are also contemplated as being within the scope of theinvention.

The foamed binder resin dispersion used to form the labile foam layerpreferably has an air content of at least 50% by volume up to 99% byvolume, a viscosity (as applied to the foraminous substrate) of at least2,000 centipoise (at about 25° C.) , and is comprised of air bubbleshaving an average size of 0.1 mm. The blow ratio preferably ranges from2:1 to 99:1, and more preferably from 15:1 to 21:1 although lower ratiosare also useful as long as the foam can support the mineral.

Preferably, the composition used as the binder dispersion that is foamedto form the labile foam and coated upon the foraminous substrate isdevoid of abrasive particles, in keeping with the objective of theinvention of temporarily delaying contact between the foraminoussubstrate and abrasive particles during processing.

In the present invention, the foamed resin composition used to preparethe labile foam is a mechanical foam or a chemical foam. Preferably amechanical foam is formed by mechanical mixing or agitation, of a gas,e.g., air, into a liquid dispersion of a binder, or alternately, thefoam can be made by passing gases under pressure (e.g., injection)through a liquid dispersion of a binder.

In the present invention, an amount of gas bubble incorporation to formthe foam is purposely induced at a level that will hold the abrasiveparticles at the surface of the foraminous substrate, by providing resinfilms between substrate surfaces, e.g., fibers, which support theparticles. The level of foaminess is such that during the curing stageor any subsequent processing stage, the supporting foam films break downas the particles become more firmly attached to the substrate, so thatfinally, the films are completely broken, and the particles are attacheduniformly upon and in the surface regions of the uppermost materialconstituting the substrate matrix.

The present invention makes it possible to concentrate abrasiveparticles at the surface regions of a three-dimensional, poroussubstrate, thus enhancing performance by allowing more particles toremain at the surface of the substrate and less particles to be embeddeddeeply into the substrate, where chances of exposure are limited or atleast delayed during abrading/scouring. The inventive method also allowsfor the use of less abrasive particulate overall, due to the higherpercentage of abrasive particles provided at the surface of thesubstrate.

In the present application, other terms, listed below, have thefollowing meanings:

"Fibrous substrate" means a self-supporting web material constituted bycontacting fibers that is porous to air.

"Nonwoven" encompasses both staple fiber webs, inclusive of random, airlaid and carded webs, spun bonded and melt blown webs, and tows formedof continuous parallel-arranged filaments.

"External surface", as used in the context of a foraminous substrateherein, means an outermost, exposed major face of the substrate.

"Abrasive particle" means a solid particle capable of removing surfacematerial from another surface when brought into inter-frictional contacttherewith.

"Hardening" means solidifying a resin by drying or curing. "Curing"means cross-linking a thermosetting resin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention relates to methods of making foraminous abrasive articleshaving high concentrations of abrasive particles bonded in the externalsurface regions thereof. More particularly, the invention uses binderdispersions applied to a foraminous substrate (support) in a foamedcondition to provide a transitory support and barrier to immediatecontact with the substrate for dry abrasive particles sequentiallydeposited thereon. The binder ultimately attaches the abrasive particlesto surface regions of the substrate upon controlled collapse of the foamand curing of the binder material mutually contacting substrate surfaceareas and abrasive particles.

After the foamy coating is applied to a surface of foraminous substrate,the abrasive particles are applied in a dry fashion. The abrasiveparticles may be any of type, shape, material, or size as long as thesurface of the substrate material, e.g. fibers, and the gross surface ofthe particles are influenced by the surface tension effects of thecoating. However, all or substantially all of the abrasive particlesshould be at least partly visible as resting on the top surface of thefoam barrier up until the time the foam is destabilized by heat and timeor pressure. Further, no significant fraction, i.e., >0.001% by wt., ofthe abrasive particles should completely sink into or become fullysubmerged into the bulk of the foam barrier layer until the foamcollapses to about 5% of its original thickness.

Curing steps are introduced to cure the coating and attach the particlespermanently to the substrate. As curing commences, the changes in thecoating due to heat cause the films of the foam to break down and fall(collapse), leaving the abrasive mineral deposited on the surface of thesubstrate fibers or other support matrix material. Subsequent curingtime gives more film breakage until the foam coating layers or films arecompletely reduced as the particles and binder harden in theirpositions.

While the foraminous substrate is occasionally exemplified herein as anonwoven web, for sake of illustration, it is to be understood that thesubstrate material is not limited thereto. For example, other forms offoraminous fibrous materials e.g., woven cloths, nonwoven batts,directional webs, and the like, are also useful substrate materials forprocessing by the invention. Moreover, non-fibrous foraminous substratesalso can be employed, such as cured, open celled synthetic foams andnatural sponge materials.

Nonwoven Webs

While the fibrous substrate of the invention can alternately be a wovencloth, or a directional web, a preferred fibrous substrate is a nonwovenweb. In this regard, web formation equipment suitable for the practiceof this invention include any such equipment capable of forming anonwoven fabric from the fibers described above. Cards, garnetts,wet-lay, and air-lay equipment may be used. Air-lay is preferred.Appropriate air-lay equipment include that known commercially as "RandoWebber", that known commercially as "Dr. O. Angleitner" or "DOA", or ahybrid system known as a "Hergeth" randomizing card. The operatingparameters for such equipment are well known to those normally skilledin the art. Nonwoven fabrics prepared using apparatus such as these haveat least two major external surfaces.

In a preferred embodiment, the fibrous substrate is an open, lofty,three-dimensional air-laid nonwoven fabric, and can be made withnonwoven webs and fiber adhesive treatments (except with no abrasiveslurry treatment) such as those described in U.S. Pat. No. 2,958,593(Hoover et al.), which is incorporated herein by reference. Alsopreferred are spunbonded, continuous filament nonwoven web constructionssuch those as described by Fitzer in U.S. Pat. No. 4,227,350, which isalso incorporated herein by reference.

As an optional enhancement to a nonwoven abrasive article made accordingto the invention, it is often desirable to promote fiber bonding withinthe nonwoven article, e.g., a nonwoven web, so that the article willhave greater structural strength and durability to better tolerateabrasive removal and cleaning environments. Conventional fiber adhesivesthat are devoid of abrasive components which are used to furtherconsolidate nonwoven webs can be used for this procedure. Such a fibertreatment can be imparted to the web as a separate treatment prior to orafter the abrasive particles are adhesively attached to external surfaceareas of the nonwoven substrate. The fiber adhesive treatment can beapplied dry, as in the form of conventional thermal-bonding shortfibers, or in liquid form using known coating or spraying techniquesfollowed by hardening and/or curing of the fiber treating or coatingmaterials in place. Embrittlement or filling of the nonwoven article bythe fiber treatment is to be avoided. Liquid form fiber treatingmaterials that do not contain abrasive grit that can be used in thisregard include those described in U.S. Pat. No. 2,958,593 (Hoover etal.), which is incorporated herein by reference.

Preferably, the fibers are bonded together at only their points ofcrossing contact. This helps to provide an open, low density, lofty webwhere the interstices between fibers are left substantially unfilled byresin or abrasive. For cleaning and scouring type applications, the voidvolume of the finished nonwoven abrasive article preferably is in therange of about 75% to about 95%. At lower void volumes, a nonwovenarticle has a greater tendency to clog-up which reduces the abrasivecutting rate and hinders cleaning of the web by flushing. If the voidvolume is too high, the web may lack adequate structure to withstand thephysical stresses associated with cleaning or scouring operationswithout rapidly failing.

The nonwoven abrasive article is provided as either a continuous web, orit can be discrete web. In making production quantities, use of acontinuous nonwoven will usually be more practical. If desired, thenonwoven article can be treated with a fiber adhesive.

Fibers

Fibers suitable for use in the fibrous substrates of the presentinvention are not particularly limited. A wide variety of fibers areuseful in a fibrous web, e.g., a nonwoven web, used as the fibroussubstrate, including both natural and synthetic fibers, and mixturesthereof. Synthetic fibers are preferred. Synthetic fibers include thosemade of polyester (e.g., polyethylene terephthalate), nylon (e.g.,hexamethylene adipamide, polycaprolactum), polypropylene, acrylic(formed from a polymer of acrylonitrile), rayon, cellulose acetate,polyvinylidene chloride-vinyl chloride copolymers, vinylchloride-acrylonitrile copolymers, and so forth. Natural fibers includethose of cotton, wool, jute, and hemp.

The fiber used may be virgin fibers or waste fibers reclaimed fromgarment cuttings, carpet manufacturing, fiber manufacturing, or textileprocessing, and so forth. The fiber material can be a homogenous fiberor a composite fiber, such as bicomponent fiber (e.g., a co-spunsheath-core fiber). Fibers may be conventionally spun or may be formedby known spunbonding or melt blowing methods, whereby the fibroussubstrate is formed as the fibers are formed.

The denier of the fiber used may vary widely, depending upon the resultsdesired. For example, heavier denier is more conducive to making coarsepads for rough scouring jobs, while lighter denier is more appropriatefor finer, less aggressive scouring jobs. The thickness of the fibers isnot particularly limited (apart from processing considerations), as longas due regard is given to the resilience and toughness ultimatelydesired in the resulting web. With the "Rando-Webber" equipment, fiberthicknesses are generally within a range of about 25 to about 250microns, corresponding to a fiber fineness or linear density of betweenabout 5 and about 500 denier.

The fibers can be curled, crimped and/or straight. However, in theinterest of obtaining maximum loft, openness and three-dimensionality inthe nonwoven article web it is preferable that all or a substantialamount of the fibers be crimped. However, crimping is unnecessary wherefibers are employed which themselves readily interlace with one anotherto form and retain a highly open lofty relationship in the formed web.

By way of example, polyamide staple fibers of tenacity between 1.0 and10.0 g/denier can be used. Fibers of a lower tenacity are too fragile toprocess through web forming machines. Fibers of tenacity higher than10.0 g/denier are expensive and are difficult to impart stable crimp.While any polyamide can be successfully incorporated into the fibroussubstrates of this invention, nylon 6 and nylon 6,6 are preferred. Nylon6,6 is most preferred. The staple length of the fibers of this inventionmay be from about 0.75 inches to 6 inches, preferably 1.0 inches to 4.0inches, most preferably 1.5 inches to 3 inches. The appropriate crimplevel (as measured full-cycle) can be between about 4 crimps/inch andabout 20 crimps/inch, preferably from about 8 crimps/inch to about 16crimps/inch. Especially useful staple fiber for the practice of thepresent invention is a 15 denier staple fiber of nylon 6,6 cut to a 1.5inch staple length, commercially available under the trade designation"Type 852" from E.I. DuPont de Nemours, Wilmington, Del.

The fibers can be used in the form of a cloth, web, a batt, or a tow. Asused herein, a "batt" is meant to refer to a plurality of webs, orsimilar structures made by air-lay methods.

Binder Dispersion

The binder composition used must be capable of being foamed. The bindercomposition can be an aqueous dispersion of a binder that hardens upondrying, such as an acrylic resin emulsion, or a dispersion of athermosetting (curing) binder. Thermosetting resins are preferred, suchas binder resins selected from among phenol formaldehyde resins,phenoplasts, aminoplasts, unsaturated polyester resins, vinyl esterresins, alkyd resins, allyl resins, furan resins, epoxies,polyurethanes, and polyimides. For example, phenolic resins suitable forthe present invention include both resole and novolak phenolic resins.Preferred is a resole-type phenolic resin comprising phenol and analdehyde, for example, a 2:1 formaldehyde:phenol composition with a NaOHcatalyst.

More preferred foamable, coatable, hardenable resin compositions areresole phenolic resins comprising a surface active agent which assistsin the formation of and enhances the stability of the resultant foam. Anexemplary surface active agent is Minnesota Mining and ManufacturingCompany FLUORAD FC-170 fluorochemical surfactant, which can be obtainedfrom the Minnesota Mining and Manufacturing Company of St. Paul, Minn.

Foaming agents (emulsifiers) or surfactants are added to the bindingresin dispersion and applied to the foraminous substrate, e.g. anonwoven web, using coating methods that are compatible with liquidcoatings. The level of surfactant or foaming agent is usually muchlarger than normally recommended for general surface tensionmodification. Amounts nearing 1.0% to 2.0% of total wet components havebeen used, compared to 0.1% recommended levels for general surfacetension modifications of coatings. By corollary, the binder dispersionof the present invention should be devoid of anti-foaming agents.

Foamable, Coatable, Hardenable Resin Composition

Foamable, coatable, hardenable resin compositions useful in the practiceof the present invention may be any that can be caused to retain itsfoam form for a sufficient length of time to allow the application ofthe abrasive particles. The resin compositions may be foamed by knownmethods, including mechanically foaming or frothing, the injection anddispersion of insoluble gas, or by the use of chemical blowing agentsthat thermally or otherwise decompose to produce a gas-phase material.Mechanical agitation is used to advantage to incorporate air into aliquid resin system (latex): such processes are sometimes referred to as"aeration" or "frothing".

For the purposes of the present invention, the foamable, coatable,hardenable resin compositions should be foamable to a blow ratio, i.e.,the ratio of foamed volume to that of the unfoamed starting material, ofbetween 2:1 and 99:1.

Upon completion of the inventive method, preferably, at least about 80%,by weight, and more preferably between 80% and 90%, by weight, ofabrasive particles deposited on the foam coating layer become attached(bonded to fibers) on or in the fibrous substrate at locations within avertical distance measured from the coated external surface that is nogreater than about 25%, and more preferably no greater than about 15%,of the overall average thickness of the fibrous substrate. The verticaldistance and thickness of the fibrous substrate are each measured in adirection normal to a horizontal plane defined by the external surfaceof the fibrous substrate. Therefore, for a nonwoven web having anoverall thickness of 10 mm, at least about 80%, by weight, of theabrasive particles applied to the upper surface of the foamed coatinglayer ultimately become bonded to the web fibers located within avertical distance of 2.5 mm from the external surface of the web thatwas coated with the foamed coating layer.

The foamed binder resin dispersion preferably has an air content of atleast 50% by volume up to 99% (or a blow ratio of between 2:1 and 99:1,more preferably between 15:1 and 21:1), a viscosity (as applied to thenonwoven web) of at least 2,000 centipoise, and is comprised of airbubbles having an average size of 0.1 mm.

The labile foam must retain its structural integrity at least until theabrasive particles are added to the composite. Otherwise, the abrasiveparticles would not have the temporary foam support that allows for theabrasive particles to be concentrated at the external surfaces of thesubstrate, such as a nonwoven web, after the temporary foam barriercollapses ("falls") when heated, e.g., in a drying oven, during curingof the resin as water and entrapped air is substantially eliminated fromthe foam.

After applying the abrasive particles to a surface of the froth or foamcoating layer on the substrate, the substrate is exposed to a heatsource, such as infrared lamps, to heat the substrate, froth to anextent necessary to collapse the foam. Heating can be done with anysource giving sufficient heat distribution and air flow. Infrared lampsare useful for applying heat in this manner.

In the case of heat-activatable thermosetting resin foams, it ispreferred that heating is sufficient to initiate curing (cross-linking)of the resin, which will cause solidification of the resin and mutualadhesion of contacted abrasive material and substrate matrix surfaces.

Preferably, the substrate, e.g., a nonwoven web, is then inverted andthe opposite surface of the substrate is coated with the foam andabrasive particles and heat-treated in the same way as the firstsurface.

Abrasive Particles

Suitable abrasive particles include those of any appropriately hardmaterial such as flint, talc, garnet, aluminum oxide, silicon carbide,diamond, silica, and a alpha-alumina ceramic material availablecommercially under the trade designation "CUBITRON" from the MinnesotaMining and Manufacturing Company of St. Paul, Minn. Abrasive particlesizes may be any but are typically from 1 micrometer or less to onemillimeter or more in dimension. Alternatively, suitable abrasiveparticles need not be inorganic materials, but may rather be syntheticmaterials such as poly(methyl methacrylate), polycarbonate, poly(vinylchloride), or other organic thermosetting or thermoplastic materialsuitably reduced to an appropriate particle size. The hardness,composition, and size of the abrasive particles are readily selectableby one of normal skill in the art, taking into consideration the natureof the workpiece to be abraded.

The abrasive particles can be drop coated, sprinkled, sprayed, and thelike, in a dry condition upon the upper surface of the foam coatinglayer, such as by conveying the substrate beneath a particle dispenser.

Suitable Converted Forms

Foraminous abrasive articles made by the present invention may take anyof a variety of conventional converted forms such as sheets, blocks,strips, belts, brushes, rotary flaps, discs, or solid or foamed wheels.Especially useful forms are discs, sheets, and wheels. The wheels aretypically in the form of a right circular cylinder having dimensionswhich may be very small, e.g., a cylinder height on the order of a fewmillimeters, or very large, e.g., two meters or more, and a diameterwhich may be very small, e.g. on the order of a few centimeter, or verylarge, e.g., one meter or more. The wheels typically have a centralopening for support by an appropriate arbor or other mechanical holdingmeans to enable the wheel to be rotated in use. Wheel dimensions,configurations, means of support, and means of rotation are well knownin the art.

Abrasive articles of larger dimensions may be made by the preparation ofmulti-layer "slabs" or "buns". Uncured or partially cured layers ofnonwoven abrasive sheet materials may be stacked, compressed and fullycured to make a layered composite structure capable of being convertedinto useful articles of substantial dimensions. This layered compositemay be used as the source of a multitude of articles of the invention,each having various diameters, or all having the same diameter, asrequired by the uses. Articles of the invention may be produced from thelayered composites by machining using appropriate techniques which arealso well known in the art. For example, a wheel shape may be die cutfrom a slab of the layered composite. Additionally, ribbons, strips, orelongate segments of a nonwoven abrasive sheet may be spirally(convolutely) wound into a wheel shape while the binder is uncured orpartially cured and then fully cured to yield a wheel.

The foraminous abrasive articles made according to this invention can beused as a cleaning, or material removing tool, or as a primary componentof such a tool.

In the following examples, objects, features and advantages of thisinvention are further illustrated by various embodiments thereof but thedetails of those examples should not be construed as limiting theinvention. All parts and percentages are by weight unless indicatedotherwise.

EXAMPLES Example 1

A low-density non-woven web weighing 147 g/m² was formed from 1.5-inchstaple of 12 denier nylon 6,6 fibers of tenacity 8.2 g/denier(commercially available under the trade designation "Type 885" from E.I.DuPont de Nemours, Wilmington, Del.) on a web-forming machine availableunder the trade designation "Rando Webber", Rando Machine Co., Macedon,N.Y. A 109 g/m² (cured weight) phenolic resin prebond coating wasapplied to the web via a two-roll coater and cured at 175° C. for about2 minutes. The resulting prebond web was then coated with a resincomposition consisting of 57.0% phenolic resin, 42.3% water, and 1.7%surfactant ("3M Fluorad FC-170" fluorochemical surfactant available fromMinnesota Mining and Manufacturing Company, St. Paul, Minn.) that wasfrothed to a labile foam form by mechanical infusion of air into theresin. The froth was formed by blending air into the liquid resin byusing a set of high speed pins (blades) in a blending chamber inequipment obtained from CSKG Industries, Inc. Reading, Penn. (alsoavailable from Gaston County Fabrication, Stanley, N.C.). The labilefoam formed was applied to the exposed face of the nonwoven web via atwo-roll coater operating at about 1.4 m/min and a nip load of 31-36kg/cm of roll width, resulting in a 209-315 g/m² wet add-on (curedcoating weight of 83-126 g/m²).

Following application of the foam coating, the upper surface of the foamcoating on the web was coated with 104 to 126 g/m² ANSI grade 280 &finer, dry alumina abrasive particles by the use of venturi outletspositioned 5 to 8 cm above the froth-containing nonwoven web. Themineral dropping apparatus was fed by venturi powder pumps which werefed from a fluidized bed. The fluidized bed was a metal box with asealed lid and powder pumps pulling fluidized mineral and air out of thetop. The powder pumps received air at 0.7 to 1.4 kg/cm² (10 to 20 psi)from a source external to the box as the means to convey the powderpulled from the fluidized bed to the ventura outlets (guns). The floorof the box was a membrane that allowed air to pass through from anotherchamber below. This lower chamber was fed with compressed air from 2.8to 5.6 kg/cm² (40 to 80 psi), depending on the mineral particle size anddensity.

The resulting web was then inverted and an identical foam coating and aparticle coating was applied to the opposite side of the web in the samemanner as the first coated side. The abrasive-coated foam/web compositewas then directed through an oven set at 175° C. providing a residencetime of 3 to 4 minutes to break the foam and cure the binder. Theresulting article was free of foamed binder and was well bonded.

The process of this example resulted in about 80%, by wt., of theabrasive particles on each web face being deposited within a depth fromthe respective exterior surfaces of the web that is equal to no morethan 25% of the thickness of the web with the innermost portion of theweb being essentially free of abrasive particles. The example alsoshowed increased coating efficiency since few particles passedcompletely through the web (which results in waste) due to the labilefoam's presence.

While the invention has been described in terms of its preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

What is claimed is:
 1. A method for making a foraminous abrasivearticle, comprising the steps of:(a) providing a foamable, hardenable,liquid resin composition and a foraminous substrate having an externalsurface; (b) foaming said resin composition to effectively disperse aplurality of gas bubbles throughout said resin composition to provide alabile foamed resin composition; (c) applying said labile foamed resincomposition to said external surface of said substrate to form a labilefoam coating layer having an exposed surface; (d) applying a pluralityof abrasive particles to said exposed surface of said labile foamcoating layer; (e) heating said foamed coating layer to effectivelyeliminate said gas bubbles from said labile foamed coating layer andprovide a resin coating including said abrasive particles; and (f)hardening said resin coating to attach said abrasive particles to saidsubstrate to form a foraminous abrasive article.
 2. The method of claim1, wherein said foraminous substrate comprises a porous materialselected from the group consisting of synthetic foam material andnatural sponge material.
 3. A method for making a fibrous abrasivearticle comprising the steps of:(a) providing a foamable, hardenable,liquid resin composition and a fibrous substrate having an externalsurface; (b) foaming said resin composition to effectively disperse aplurality of gas bubbles throughout said resin composition to provide alabile foamed resin composition; (c) applying said foamed resincomposition to said external surface of said fibrous substrate to form alabile foam coating layer having an exposed surface; (d) applying aplurality of abrasive particles to said exposed surface of said labilefoam coating layer; (e) heating said foamed coating layer to effectivelyeliminate said gas bubbles from said labile foam coating layer; andprovide a resin coating including said abrasive particles and (f)hardening said coating composition to attach said abrasive particles tosaid fibrous substrate to form a fibrous abrasive article.
 4. The methodof claim 3, wherein at least about 80%, by weight, of said abrasiveparticles applied to said foamed coating layer in step (d) becomeattached to said fibrous substrate upon completion of step (f) within avertical distance measured from said external surface that is no greaterthan about 25% of overall average thickness of said fibrous substrate.5. The method of claim 3, wherein said foamed resin composition has anair content of at least 50% by volume up to 99% by volume uponcompletion of step (b).
 6. The method of claim 3, wherein said foamedresin composition has a blow ratio of between 15:1 to 21:1 uponcompletion of step (b).
 7. The method of claim 3, wherein said gasbubbles comprise air bubbles.
 8. The method of claim 3, wherein said gasbubbles have an average size of 0.1 mm.
 9. The method of claim 3,wherein said foamed resin composition is devoid of abrasive particles.10. The method of claim 3, wherein said fibrous substrate comprises anonwoven substrate.
 11. The method of claim 10, wherein said nonwovensubstrate is selected from the group consisting of a nonwoven web, anonwoven batt, and a nonwoven tow.
 12. The method of claim 3 whereinsaid fibrous substrate comprises organic fibers selected from the groupconsisting of natural fibers, synthetic fibers, and mixtures thereof.13. The method of claim 3, wherein said fibrous substrate comprisesorganic fibers selected from the group consisting of polyester,polyamide, polypropylene, acrylic, rayon, cellulose acetate,polyvinylidene chloride-vinyl chloride copolymer, vinylchloride-acrylonitrile copolymer, and mixtures thereof.
 14. The methodof claim 3, wherein said foaming of step (b) comprises mechanicalfoaming.
 15. The method of claim 3, wherein said foaming of step (b)comprises mechanical agitation of said resin composition.
 16. The methodof claim 3, wherein said foaming of step (b) comprises injection of gasthrough said resin composition.
 17. The method of claim 3, wherein saidresin composition comprises a binder resin selected from the groupconsisting of phenol formaldehyde resins, phenoplasts, aminoplasts,unsaturated polyester resins, vinyl ester resins, alkyd resins, allylresins, furan resins, epoxies, polyurethanes, and polyimides.
 18. Themethod as in claim 3, wherein said abrasive particles are selected fromthe group consisting of aluminum oxide, coal slag, flint, siliconcarbide, garnet, silica, talc, glass, metal particles, and granite.